Actually to TS why don't you make a trip down to your neighbourhood sports stadium and take a look at the lighting with your current gear and roughly work out what focal length you need and what aperture you need and what is the max ISO you can go. I think it might give you more information than is giving suggestions and maybe not all of us shoot sports under this situation.
For your budget it would probably be canon 70-200 f2.8 mark i (used), or sigma / tamron's equivalent. Smaller aperture but longer focal length may or may not work out for you as you have to see if you can get sufficient shutter speed at higher iso while having usable IQ.
You may have chance to get a 2nd hand 300 f2.8 by sigma or tamron.